The invention relates generally to a novel method of blow molding and fluorinating plastic containers and, more particularly, it relates to a method of fabricating fuel tanks for automotive vehicles which is safe for personnel operating the molding machine and which satisfies modern environmental protection requirements.
It is well known to produce plastic containers by blow molding processes and to impart a substantially impervious skin surface to such containers by treating them with a fluorine containing gas. A preferred material for manufacturing such containers is high-density polyethylene having a density, for instance of from about 0.935 to about 0.965, and a melt index in the range of from about 0.05 to about 5.0. It is also known to treat such containers with a gas containing fluorine in such a manner that fluorine remains in the wall surface of the containers in a quantity of from about 0.01 to about 30 .mu.g of fluorine per cm.sup.2 of wall surface.
Neither the materials from which containers made in accordance with the invention are made (e.g. high density polyethylene, or copolymers thereof, as well as mixtures containing copolymerizable monomers) nor the admixtures of gasses employed for the blow molding process form part of this invention. It is believed that a person of ordinary skill in the art of blow molding automotive fuel tanks is sufficiently familiar with such materials.
Reference may, however, be had to west German Patent No. specification DE-PS 24 01 948, disclosing a method of blow molding containers utilizing pressurized inert gas containing from 0.1 to 10% by volume of fluorine. The method there described, while providing containers shaped to conform to the shape of the mold cavity and having an interior skin surface, as a result of the fluorine in the blow gas, which is substantially impervious to certain volatile fluids, no consideration appears to have been given to the danger resulting from the toxicity and corrosiveness of fluorine. West German Patent specification No. DE-AS 19 95 984, relating to blow molding of automotive fuel tanks from polyethylene blanks in the presence of a fluorine containing blow gas lacks any teaching in respect of precautionary measures in the handling of the toxic and corrosive fluorine. Also, U.S. Pat. No. 2,811,468 and west German Patent specification Nos. DE-PS 26 44 508 and DE-OS 29 24 797 while treating generally methods of blow molding containers in the presence of fluorine gas do not teach anything regarding possible ways of reducing potential dangers resulting from the use of fluorine.
However, when using hydraulically actuated molds in the process of manufacturing containers of the kind here under consideration, there exists an acute risk or potential danger that fluorine may unintentionally escape during opening or closing of the mold, for instance were a container is leaking, or in case of malfunction of the hydraulic closure mechanism.
It is, therefore, a primary object of the present invention to provide a novel method of blow molding plastic containers in the presence of fluorine.
Another object of the invention resides in a method of blow molding fuel containers having a skin surface impervious to liquid fuels such as gasoline or Diesel oil.
A further object of the invention is to provide a method of blow molding fuel tanks which prevents fluorine from escaping a blow mold except when intended.
Yet another object of the invention resides in a novel method of operating a blow mold in the manufacture of fuel tanks made from polyethylene blanks and utilizing fluorine gas for providing a fuel impervious skin surface.
Furthermore, it is an object of the present invention to provide a novel blow molding method for manufacturing automotive fuel tanks which effectively protect personnel operating the mold from potential harm which may otherwise result from the escape of fluorine gas.